Wet avalanches have closed the east entrance road over Sylvan Pass in Yellowstone. NPS photos.

A combination of warmer weather and a heavy snowpack has generated a series of avalanches that have closed the east entrance road in Yellowstone National Park.

While the park's north and west entrances are open, and the south entrance was scheduled to open today, the wet snow avalanches have left more than 20 feet of snow on the road that crosses Sylvan Pass from the park's east entrance.

"Four significant slides in the pass – one resulting in a debris field 70 yards wide and 20-30 feet deep across the road – have occurred in the past 36 hours," park officials said Thursday afternoon. "Consecutive days of mild spring temperatures continue to deteriorate high-elevation snowpack conditions and are expected to delay the reopening of Sylvan Pass for an indeterminate time until the safety of motorists can be assured."

To accelerate the reopening the road park officials redirected heavy road clearing equipment to support those already at work digging away at the deep snow. Avalanche crews also searched the slide area Thursday with probes and canine rescue teams to ensure no motorists were caught in the slide.

An unoccupied government vehicle did sustain damage when it was partially buried in a major slide as a ranger was conducting an assessment of the area on foot May 11. The ranger was uninjured in the incident.

Explosives also were used Thursday to try and bring down some of the heavy, wet snow, a park release said. Thirteen of 18 of the detonations were successful in releasing large amounts of snow.

"This snow slide activity is expected to continue until the weather pattern returns to freezing night time temperatures. The current forecast for the Sylvan Pass area is for daytime temperatures in the 50s over the next two to three days, which will continue to warm the heavy snowpack and make it increasingly unstable," a park release said. "Overnight lows in the past 48 hours have dipped just enough below freezing to create a thin layer of ice, but that crust melts quickly by midday."

A blanket of heavy melting snow in the park’s interior has also contributed to the roof collapse of the RV repair facility at Fishing Bridge and caused roof damage to roof of the Grant Village Visitor Education Center. No injuries were reported in either incident, and repair work is under way.

Evidence of the park's struggles from winter to spring can be seen at Mammoth Hot Springs, where some green grass has been appearing, while other areas of Yellowstone remain wrapped in deep snow and chillier temperatures. Snow and ice still present in road turnouts and on thermal area boardwalks will make walking difficult or impossible for several more weeks, park officials said.